Modern high speed bipolar junction transistors (BJTs) are generally constrained by the competing concerns of high beta (i.e., forward current gain) and high base punch-through resistance.
Punch-through refers to the effect wherein the neutral base width is reduced to zero at a sufficiently high collector-base voltage, V.sub.cb. With the neutral base reduced to zero, the collector-base depletion region is in direct contact with the emitter-base depletion region. At this point, the collector is effectively short-circuited to the emitter, and a large current flows.
The effects of beta and punch-through voltage are both coupled directly through the base charge, Q.sub.b. The total charge in the base region of a bipolar transistor is given by the product of the doping concentration, N.sub.A, and the base width, W.sub.b. Lowering Q.sub.b raises beta but also lowers punch-through voltage at the same time. For the opposite condition, raising Q.sub.b lowers the beta that resulting in a higher punch-through voltage. The trade-off between high beta and high punch-through resistance has been a fundamental feature of silicon bipolar junction transistors for many years.
In the past, researchers have attempted to decouple the competing effects of beta and punch-through voltage by altering the band gap characteristics of the emitter, the base, or both. Lowering the band gap at the base, for instance, by the use of silicon germanium (SiGe) alloys has produced favorable beta values with acceptable punch-through voltages. Alternatively, experiments in utilizing silicon carbide (SiC) alloys to raise the band gap of the emitter region have also produced favorable results. Considerable work is being done today in the technical community to further increase the performance of these "band gap engineered" structures.
While efforts to engineer the band gap of the emitters and base regions have produced impressive results, these results have not come without certain costs. One of the primary drawbacks of using alloy semiconductors such as silicon germanium or silicon carbide is that forming such alloys generally require the use of exotic processing equipment. Typically, heterojunction epitaxial layers are grown using a technique known as molecular beam epitaxy (MBE). In this method the substrate is held in a high vacuum while molecular or atomic beams of the constituent atoms impinge upon its surface.
The main problem with MBE machines, however, is that they are characterized by extremely slow growth rates (approximately 1 micron/hr.) and are very expensive and very difficult to operate in a manufacturing environment. Moreover, techniques such as MBE for forming heterojunction alloys are generally not compatible with modern processing requirements and structures, (e.g., BiCMOS processes). In light of these limitations, advanced spitaxial growth techniques like MBE have been limited to research facilities or to specific applications (e.g., microwave amplification devices) where the level of integration is severely limited and the manufacturing volumes are likewise small. Thus, the goal of simultaneously achieving high beta and high punch-through voltage in a bipolar transistor manufactured using conventional silicon processing equipment and techniques has not yet been accomplished.
As will be seen, the present invention provides a solution to the problem of achieving high beta and simultaneously high punch-through voltage in a homojunction bipolar device. Furthermore, according to the present invention, the novel device structure is easily fabricated using standard silicon processing equipment and lithographic techniques. Moreover, the invention is well-suited to a large volume, high production semiconductor manufacturing environment.